SQL UNION

The purpose of the SQL UNION query is to combine the results of two queries together. In this respect, UNION is somewhat similar to JOIN in that they are both used to related information from multiple tables. One restriction of UNION is that all corresponding columns need to be of the same data type. Also, when using UNION, only distinct values are selected (similar to SELECT DISTINCT).

The syntax is as follows:

[SQL Statement 1]
UNION
[SQL Statement 2];

Say we have the following two tables,

Table Store_Information

Store_Name

Sales

Txn_Date

Los Angeles

1500

Jan-05-1999

San Diego

250

Jan-07-1999

Los Angeles

300

Jan-08-1999

Boston

700

Jan-08-1999

Table Internet_Sales

Txn_Date

Sales

Jan-07-1999

250

Jan-10-1999

535

Jan-11-1999

320

Jan-12-1999

750

and we want to find out all the dates where there is a sales transaction. To do so, we use the following SQL statement: